1. Field of the Invention
The present invention relates to a structure of a transmission for a bicycle including a derailleur. In particular, the present invention relates to an improved structure of a transmission for a bicycle characterized by the mounting structure of the derailleur in the transmission.
2. Description of Related Art
In the related art, a transmission for a bicycle provided with a derailleur is known. The derailleur has a supporting arm with a pantograph mechanism, which is adapted to shift a power transmission chain wound around a multi-stage sprocket wheel supported by a rear wheel hub and mounted to the rear wheel hub via a shaft step-by-step is known. The derailleur is provided with a guiding ring for shifting the chain and a tension ring for providing tension to the chain. The supporting arm, which is formed of a pantograph mechanism having a pair of links, supports a derailleur shaft of the derailleur. This support of the derailleur shaft has a substantially cantilevered structure (for example, see JP-A-59-2986, pages 1-2 and FIGS. 1-2).
The above-described document discloses a derailleur 05 of a transmission for a bicycle. The derailleur 05 has a supporting arm 04 of a pantograph mechanism, which is adapted to shift a power transmission chain 03 wound around a multi-stage sprocket wheel 02 supported by a rear wheel hub 01 and mounted to the rear wheel hub 01 via a shaft step-by-step. The derailleur 05 includes a shifting frame 09 having a guide ring 07 supported to the distal end of the supporting arm 04 of a pantograph type via a movable member 06 for shifting the chain. A tension ring 08 provides tension to the chain. The shifting frame 09, which constitutes a substantial body of the derailleur 05, is cantilevered by the movable member (derailleur shaft) 06.
According to the cantilever supporting structure of the derailleur (shifting frame) as disclosed in the above document, sufficient rigidity of the derailleur shaft (movable member) is difficult to secure. In order to secure sufficient rigidity, it is necessary to enforce the rigidity of the derailleur shaft itself. In order to do so, countermeasures such as increasing the thickness of the derailleur shaft have to be taken. However, when such a countermeasure is taken, increase in weight is inevitable, whereby upsizing of the derailleur shaft mounting structure itself cannot be avoided.